Add Mercury topography data

[leouieda]

There are both line data and gridded products on the NASA PDS.

• Line data: https://pds-geosciences.wustl.edu/messenger/mess-e_v_h-mla-3_4-cdr_rdr-data-v2/messmla_2101/rdr_radr/ (column descriptions)
• Grids: https://pds-geosciences.wustl.edu/messenger/mess-e_v_h-mla-3_4-cdr_rdr-data-v2/messmla_2101/gdr/img/ (format is kind of strange really)

The line data would be good for spherical interpolation tests and cross-over analysis if we ever get into that.

[leouieda]

The FAQ say that everything there is public domain: https://pds-geosciences.wustl.edu/faq.html